1. Field of the Invention
The present invention relates to a method and apparatus of manufacturing a negative electrode for a non-aqueous electrolyte secondary battery, and particularly to a technique suitable for mass production for controlling an amount of lithium to be stored in the negative electrode so that the irreversible capacity thereof can precisely be compensated for.
2. Background Art
The development of applications of a non-aqueous electrolyte secondary battery using lithium as main power supply for various kinds of devices is proceeding because of a large theoretical capacity of lithium. Under circumstances where further increases in the capacity is required, for the negative electrode active material, changeover is intended from carbon materials, e.g. graphite, having a theoretical capacity lower than 400 mAh/g to silicon- or tin-containing materials having a theoretical capacity of substantially ten times.
Substantially all the negative electrode active materials containing carbon material have a capacity loss caused by non-dischargibility (irreversible capacity) in the initial charge/discharge. In particular, it is known that large-capacity materials containing silicon, tin or the like have larger irreversible capacity. It is considered that the irreversible capacity of a negative electrode active material results from deactivation of lithium caused by side reactions thereof with the components of the electrolyte solution or negative electrode active material during charging. Because the irreversible capacity resulting from the negative electrode active material finally causes a partial loss of the reversible capacity of the positive electrode, i.e. a capacity limiting electrode, the entire capacity of the non-aqueous electrolyte secondary battery is decreased.
To prevent decreases in the battery capacity resulting from the irreversible capacity, some techniques are proposed. In the techniques, an amount of lithium is added to the negative electrode active material in advance to compensate for the irreversible capacity. For example, Japanese Translation of PCT Publication No. 96/027910 discloses a method of attaching foil made of metal lithium to a negative electrode sheet of a complex oxide containing tin by rolling transfer or the like. In this method, an electrolytic solution allows the negative electrode active material to store lithium in the subsequent battery assembling step.
However, the amount of lithium to be stored in the negative electrode active material is much smaller than the amount of lithium to be imparted according to the lower limit (approximately 30 μm) of the thickness of metal lithium foil that can be handled. For this reason, the metal lithium foil is partly provided on the negative electrode in this method, and lithium cannot be stored therein uniformly. Thus, it causes expansion which deforms the negative electrode and makes the charge/discharge reaction non-uniform.
Japanese Patent Unexamined Publication No. 2005-038720 discloses a method of forming a light metal layer such as metal lithium on a mixture layer containing a negative electrode active material, using a film forming method in a dry process, such as vacuum deposition. In this method, the treated negative electrode is placed in a dry atmosphere or electrolyte solution so that lithium is stored in the negative electrode.
With this method, a lithium layer thinner than lithium foil can be formed by vacuum deposition. However, if some factors cause the thickness of the lithium layer to deviate from a predetermined value, the deviation cannot be known. In such a case, negative electrodes having a large deviation from the predetermined value are continuously produced.
Further, Japanese Patent Unexamined Publication No. H06-325765 proposes a technique for fabricating another electrochemical cell prior to the assembly of a battery, and charging the negative electrode using a counter electrode capable of releasing lithium.
With this method, the amount of lithium to be imparted to the negative electrode active material can accurately be measured from the amount of current through the electrochemical cell. However, in the method using such an electrochemical cell, continuous production is difficult. Thus, this method is unsuitable for mass production.